In order for the human eye to have clear vision of objects at different distances, the effective focal length of the eye must be adjusted to keep the image of the object focused as sharply as possible on the retina. This change in effective focal length is known as accommodation and is accomplished in the eye by varying the shape of the crystalline lens. Generally, in the unaccommodated emmetropic eye the curvature of the lens is such that distant objects are sharply imaged on the retina. In the unaccommodated eye near objects are not focused sharply on the retina because their images lie behind the retinal surface. In order to visualize a near object clearly, the curvature of the crystalline lens is increased, thereby increasing its refractive power and causing the image of the near object to fall on the retina.
The change in shape of the crystalline lens is accomplished by the action of certain muscles and structures within the eyeball or globe of the eye. The lens is located in the forward part of the eye, immediately behind the pupil. It has the shape of a classical biconvex optical lens, i.e., it has a generally circular cross section having two convex refracting surfaces, and is located generally on the optical axis of the eye, i.e., a straight line drawn from the center of the cornea to the macula in the retina at the posterior portion of the globe. In the unaccommodated human eye the curvature of the posterior surface of the lens, i.e., the surface adjacent to the vitreous body, is somewhat greater than that of the anterior surface. The lens is closely surrounded by a membranous capsule that serves as an intermediate structure in the support and actuation of the lens. The lens and its capsule are suspended on the optical axis behind the pupil by a circular assembly of very many radially directed elastic fibers, the zonules, which are attached at their inner ends to the lens capsule and at their outer ends to the ciliary body and indirectly to the ciliary muscle, a muscular ring of tissue, located just within the outer supporting structure of the eye, the sclera. The ciliary muscle is relaxed in the unaccommodated eye and therefore assumes its largest diameter. According to the classical theory of accommodation, originating with Helmholtz, the relatively large diameter of the ciliary muscle in this condition causes a tension on the zonules which in turn pulls radially outward on the lens capsule, causing the equatorial diameter of the lens to increase slightly and decreasing the anterior-posterior dimension of the lens at the optical axis. Thus, the tension on the lens capsule causes the lens to assume a flattened state wherein the curvature of the anterior surface, and to some extent the posterior surface, is less than it would be in the absence of the tension. In this state the refractive power of the lens is relatively low and the eye is focused for clear vision for distant objects.
When the eye is intended to be focused on a near object, the ciliary muscles contract. According to the classical theory, this contraction causes the ciliary muscle to move forward and inward, thereby relaxing the outward pull of the zonules on the equator of the lens capsule. This reduced zonular tension allows the elastic capsule of the lens to contract causing an increase in the antero-posterior diameter of the lens (i.e., the lens becomes more spherical) resulting in an increase in the optical power of the lens. Because of topographical differences in the thickness of the lens capsule, the central anterior radius of curvature decreases more than the central posterior radius of curvature. This is the accommodated condition of the eye wherein the image of near objects falls sharply on the retina.
Presbyopia is the universal decrease in the amplitude of accommodation that is typically observed in individuals over forty years of age. In the person having normal vision, i.e., having emmetropic eyes, the ability to focus on near objects is gradually lost, and the individual comes to need glasses for tasks requiring near vision, such as reading.
According to the conventional view the amplitude of accommodation of the aging eye is decreased because of the loss of elasticity of the lens capsule and/or sclerosis of the lens with age. Consequently, even though the radial tension on the zonules is relaxed by contraction of the ciliary muscles, the lens does not assume a greater curvature. According to the conventional view, it is not possible by any treatment to restore the accommodative power to the presbyopic eye. The loss of elasticity of the lens and capsule is seen as irreversible, and the only solution to the problems presented by presbyopia is to use corrective lenses for close work, or bifocal lenses, if corrective lenses are also required for distant vision.
Contrary to the conventional view, it is possible to restore the accommodative power to a presbyopic eye by implanting a plurality of scleral prostheses within the sclera of the eye. For each individual scleral prosthesis an incision is made in the sclera of the globe of the eye near the plane of the equator of the crystalline lens. The incision is then extended under the surface of the sclera to form a scleral “pocket.” The scleral prosthesis is then placed within the pocket. A typical scleral prosthesis comprises a generally rectangularly shaped bar approximately five millimeters (5.0 mm) long, one and one half millimeters (1.5 mm) wide, and one millimeter (1.0 mm) tall. The anterior edge of the scleral prosthesis applies an outward force on the anterior edge of the scleral pocket which elevates the anterior portion of the sclera attached thereto and the ciliary body immediately beneath the sclera to increase the working distance of the ciliary muscle. This method is described more fully in the “Presbyopia and Related Eye Disorder Patent Documents” that have been incorporated by reference into this patent document.
A physician who makes the incisions to form a scleral pocket must be a very skilled surgeon. The surgeon must use great care to ensure that the incisions are made properly. The incisions that must be made to form a scleral pocket are quite small. The incisions must be made at precisely the correct depth. The width and length of the scleral pocket must also be formed by very precise incisions.
It is well known that physicians may differ significantly with respect to the level of surgical skill that they possess. Physicians who practice surgery regularly generally become quite skilled. Other physicians who do not practice surgery regularly are less skilled. Even skilled surgeons may find it difficult to make the precise incisions that are required to correctly form a scleral pocket.
If scleral pocket incisions are not made with sufficient precision the resulting scleral pocket will not be able to correctly support a scleral prosthesis. An incorrectly supported scleral prosthesis is not able to provide an acceptable level of vision correction.
A scleral pocket must be located on the sclera with sufficient precision to ensure that a scleral prosthesis that is placed within the scleral pocket will be able to function correctly. An incorrectly located scleral pocket will not enable a scleral prosthesis to provide an acceptable level of vision correction.
It would be desirable if a system and method existed that would allow a surgeon to precisely locate an optimal position for forming a scleral pocket within the sclera of an eye. Accordingly, a need exists in the art for a system and method that is capable of precisely locating a position on the sclera of an eye to form a scleral pocket to receive a scleral prosthesis.